Identification of cation and oxidic caesium species in basic Cs-overloaded BEA zeolites

A parent acidic H-BEA with crystallites very small in size and high external surface area was used to prepare a series of materials loaded with increasing Cs+ contents by firstly ion-exchange and then impregnation with CsOH solutions. The monitoring of the ion- exchange process by chemical analysis and by IR spectroscopy in presence of CO or NH3 reveals that a relevant amount of Brønsted acid sites in dehydrated H-BEA is related to framework Al sites that, in aqueous solution, turn into partially extraframework Al species unable to act any longer as sites of cationic exchange. This limits the exchange capacity in solution and higher levels of ion-exchange are attained by subsequent impregnation and calcination. A possible explanation for such a behaviour is proposed. The formation of car- bonates by adsorption of CO2, monitored by IR, confirms that the basic character induced on framework oxygen atoms by exchange of H+ with Cs+ is significantly weaker than that reached upon Cs-overloading. For the latter, the strong basicity is related to the presence of Cs2O-like nanoparticles (also detected by EXAFS), dispersed within the zeolite pores (as shown by pore volume and TEM/EDX mea- surements). IR spectroscopy of adsorbed CO shows that Cs+ as countercations or as surface sites of occluded Cs2O-like species exhibit a similar Lewis acid strength. Noticeably, in Cs-overloaded BEA, pairs of Cs+ sites (formed by two countercations and/or one counter- cation and a Cs+ at the surface of Cs2O-like particles) are present, where CO can be adsorbed in a head–tail form, producing a distinct mCO band at 2145 c

Organometallic approach for the synthesis of nanostructures

1144-0546Nanostructures are considered as chemical systems of high potential owing to their unusual properties at the interface of those of molecular species and bulk metals. Consequently, they are promising candidates for application in different domains such as catalysis, magnetism, medicine, opto-electronics or sensors. The control of the characteristics of nanostructures is a fundamental prerequisite if one envisages exploring their physical or chemical properties since they vary dramatically with size, shape and surface state. Thus, the development of efficient methods leading to reproducible nanostructures is presently one of the main objectives in the nanochemistry community. Although organometallic chemistry has been early involved, it arises only marginally in the field. Nevertheless, the concepts and techniques of organometallic chemistry appear to be well-adapted for the growth of well-controlled nanostructures. This will be discussed through recent advances in the synthesis of metal and metal oxide nanoparticles in terms of size dispersion, chemical composition, surface state, shape or organization, pointing out the role of ligands. Moreover their characterization at a molecular level and the development of their chemical/physical properties towards applications will be described. This review reflects more than 20 years of efforts of our team to achieve these goals